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Introduction

Protein turnover measurements by stable isotope techniques can be applied to assess
the nutritional/metabolic status in critically ill patients and their response to
feeding. Because of uncertain gastrointestinal transport and uptake of nutrients,
their contribution needs to be measured separately. We studied whole-body protein
kinetics, with special emphasis on the contribution of dietary protein, in ICU patients
and healthy controls.

Methods

Mechanically ventilated, not enterally fed ICU patients (n = 9) were recruited from an interdisciplinary ICU. Healthy, overnight-fasted volunteers
(n = 6) served as reference. A primed constant i.v. infusion of 2H-labeled phenylalanine (Phe) and tyrosine was used to quantify whole-body protein
metabolism. Patients remained on parenteral nutrition (PN) as clinically indicated;
controls received PN starting 2.5 hours before starting enteral feeding. Intrinsically
13C-Phe-labeled casein was infused for 6 hours by nasogastric tube at 0.75 g protein/
hour, together with maltodextrin at 2.73 g/hour. Protein breakdown, synthesis, net
balance, and Phe splanchnic extraction were calculated before and at the end of the
enteral feeding period, using equations for steady-state whole-body protein kinetics.
Comparisons were made by Wilcoxon matched pairs and Mann-Whitney U tests; values are
reported as mean ± SD.

Conclusion

Intrinsically isotope-labelled casein can be used to quantify dietary contribution
to protein metabolism in critically ill patients. Hypocaloric enteral feeding marginally
improved protein balance in these patients. The low recovery of enterally administered
labelled amino acid underlines the need to quantify uptake from the gastrointestinal
tract when protein turnover measurements are performed in critically ill patients
on enteral nutrition.